Memory devices are often categorized by their ability to retain data over time. For example, memory devices are considered to be volatile if they retain data while powered and, conversely, the data is lost when power is no longer provided. Static random access memory (SRAM) and dynamic random access memory (DRAM) are examples of volatile memory devices. Although losing data in response to a power loss is disadvantageous, such volatile memory devices permit relatively fast random data access (i.e., reading and/or writing).
Another category of memory device is referred to as non-volatile because they store information even after the memory is disconnected from a power source. Non-volatile memory devices typically provide slower data access and are exemplified by the read only memory (ROM), the electrically erasable programmable read only memory (EEPROM), and the FLASH memory. FLASH memory devices are used in a variety of applications including, for example, personal computers, cell phones, memory sticks, personal digital assistants (PDAs), digital cameras, microcomputer chips, wireless transmitters and receivers, and smart cards.
There have been ongoing efforts to design memory devices that are non-volatile but that also realize the access-time advantages of the volatile type. Many of these efforts have been driven by ongoing needs to reduce the size of the memory devices (i.e., the size of the cells within and/or feature sizes thereof), to reduce power requirements, and/or to increase data-access time. With regards to size reduction, for example, current semiconductor manufacturing efforts can implement the smallest features using 65 nm technology, and ongoing physical scaling efforts for such memory devices are expected to realize their physical scaling limit in the range of the 45 nm to 32 nm technology node. There are several candidates for less-volatile (and/or non-volatile) memory devices that realize one or more of these advantages. Among these candidates is the resistance-change memory.
The nonvolatile resistance-change memory typically stores information as logic bits (logic “1” and logic “0”) as a “high” or a “low” resistance state of the memory cell. This type of memory includes a electrochemical material separating two electrodes (e.g., a metal-insulator-metal structure), and a bit of data is stored and/or altered by changing the conductivity of the electrochemical material so that the resistance between the electrodes exhibits at least two different states, including a high-resistance state and a low-resistance state.
A resistance-change memory (RCM or RRAM), which can also be referred to as phase-change RAM or an electrochemical RAM, operates based on the intrinsic formation of a conductive filament-like region in the electrochemical material. In such filamentary memories, the resistance change effect is local and situated close to or inside the filament-like region. However, the filamentary nature of the memory effect itself is a disadvantage of all filamentary based memories, since this memory class does not scale well in relation to area or thickness. It remains therefore questionable, whether filamentary-based memories can be commercialized. A disadvantage of PCRAM is the large current consumption of the memory during program/erase operations.
At a more detailed level, such filamentary-based memories are commonly composed of a metal-insulator-metal structure. The insulator (or dielectric) is a binary oxide (e.g., NiO2, TiO2 or AL2O3), a more complex oxide (e.g., perovskites ABO3, SrTiO3, SrZrO3, BaTiO3 or BaSrTiO3), or chalcogenide (e.g., Cu2S, AgS or ZnCdS). In terms of its operation, initially, the insulator exhibits a high resistance and does not show any memory effect. Before the insulator is used as a memory device (e.g., NVRAM), a conductive filament-like region is formed within the insulator by a high-voltage forming process. The filament-like region is developed by applying a voltage close to the break down voltage across the insulator for an extended period of time. This process is also referred to as “the formation process” or “the forming process.”
The filament-like region can be regarded as an incomplete local breakdown of the material. In many cases, a current compliance on the programming pulse has to be used to prevent a complete breakdown. If and where the filament-like region forms is strongly correlated with the location and number of crystallographic defects in the electrochemical material. As a result, filament-like regions are often randomly distributed in the electrochemical material.
Once the filament-like region is formed, the memory effect of resistance-change memory is based on a local redistribution of mobile ions or ion vacancies by a voltage pulse of positive or negative polarity (program or erase pulse) via the electrodes. A change in the ion spatial distribution is related to a change of the electronic conductivity. A variation of the electrode area shows that the resistance change in these devices is a local effect and that the effect is coupled to the conductive filament-like region(s).
The resistance change of the memory is caused by a local redistribution of ions or ion vacancies. Mobility of ions and/or ion vacancies in binary and complex oxides is thermally activated. At room temperature, the mobility of ions and vacancies is low so that large fields close to the dielectric break down field are needed to change the ion profile. At elevated temperatures of 150° C. to 300° C., the mobility has increased and ions can easily be moved at least small distances (in terms of nanometers) within μs time scales and under significantly reduced electric fields.
Due to its low resistance, the conductive filament-like region can dissipate significant heat. During program and erase operation, the filament-like region acts as a resistive heater, which causes a self-heating of the memory cell. The filament-like region provides thermal energy for the migration of ions in the vicinity of the filament and may also provide an easy diffusion path for ions at program/erase temperatures.
One advantage of filamentary type memories is good data retention. Both high and low resistance states are stable for several years (typically at least 10 years) and can be readout without losing the stored information (i.e., non-destructively).
Such filament-based memories present a number of challenges to be overcome before successful commercialization. Their requirement of a forming process is an impediment to the manufacturing process. These memories also exhibit an uncontrolled filament-like resistance, a drift of the filament resistance with the number of program/erase cycles (aging of the cell), and an undefined active memory area in the memory cell. Further, at least partly due to their undefined active memory area in the cell, these devices have exhibited a poor scaling behavior. In addition, mainly due to a change of the filament-like region resistance with ongoing usages of the devices, their implementations have been disadvantaged by a shift in the program/erase voltage, a drift in the “on”-resistance and the “off”-resistance, and low-cycling endurance. Various changes of the electrical properties of the filament-like region, perhaps attributable to its random formation and nature (one dimensional lattice defect), present further challenges.